Liquid-crystal displays, or LCDs, found in televisions, computers, and cell phones, are very inefficient: their complex optical layers discard over 90 percent of the light they produce internally, some of it because it’s not quite the right color. Displays that will be in products made by Korean electronics company LG at the end of the year will have a better color gamut and save battery life by using more of the light that normally gets tossed out.
The displays incorporate nanomaterials called quantum dots that convert light from the backlight into narrowly defined bands of color that are matched to the display’s filters. Depending on the design of the display, the addition of quantum dots made by Palo Alto, CA-based company Nanosys improves power efficiency by more than 10 percent and significantly improves the color gamut of the display. LG demonstrated a cell-phone-sized display incorporating the quantum-dot technology last week at the Society for Information Display’s annual meeting in Seattle. The company has not yet announced what particular product the quantum-dot backlight will be used in first.
“LCDs are very inefficient, and there has not been much improvement in them over decades,” says Paul Semenza, a senior analyst at research firm Display Search. All of the major display manufacturers are working on technologies for improving the efficiency of LCDs, particularly for portable electronics like e-readers and cell phones, where battery life is paramount.
One source of inefficiency in these displays is the backlight itself. Because the optics inside LCDs toss out so much light, the backlight has to be very bright to create a good picture. “You go to the trouble to create white light,” says Semenza, “but then you have color filters that block most of it out.” Some displays get around this problem by using red, blue, and yellow light-emitting diodes (LEDs) rather than a white fluorescent lightbulb. But this is expensive, and not all LEDs are created equal: blue LEDs are much more efficient at converting electricity into light. Coating blue LEDs with a phosphorescent material that converts some of the light into yellow, red, and green, however, has the same drawback as using a white light source: most of that light is tossed out by the filters.
Nanosys has developed an add-on for blue LED display backlights that converts some of the blue light into red and green light of narrowly defined wavelengths selected to match the LCD’s filters. The company’s “quantumrail,” a thin capillary that can be attached to a backlight, contains a suspension of quantum dots that convert the light.
Quantum dots get their name from their unusual properties: when structured at the nanoscale, the optical and electronic properties of certain semiconducting materials such as cadmium are dictated by their dimensions. Conventional semiconducting materials emit light of a particular color when they’re bombarded with electrons or photons–this is how light-emitting diodes work. By carefully controlling the dimensions of quantum dots at the nanoscale, it’s possible to precisely tune what color light they emit.
Researchers have been making quantum dots since the 1980s, but it’s only this year that these nanomaterials have been incorporated into consumer products. MIT spinoff QD Vision was the first to market with a consumer product. Its quantum dots are incorporated into energy-efficient LED lighting made by Nexus. They convert light from an LED into a mixture of colors that’s more pleasing to the eye. Company CTO Seth Coe-Sullivan says QD Vision is also working with major display companies to incorporate quantum dots into LCD backlights. Coe-Sullivan says these products will launch next year.
The displays incorporating Nanosys’s quantumrail that were exhibited at the conference in Seattle had a better color gamut than traditional LCDs. A good notebook display can generate 72 percent of the colors dictated by a commonly used measurement of color gamut called the National Television System Committee standard, but the LG display rates 103 percent–that is, it can show colors that aren’t included in this standard. Running the display at lower power to create a 72 percent color gamut can add 10 percent to the battery life, according to Nanosys.